Lemma 7.1 (Glazman's Lemma [1]}, [2]})  
For all \(\lambda \in \mathbb {R}\) , we have
\(N^\pm (A;\lambda ) = & \max \left\lbrace  \dim F; \ F\in {F}^\pm (A;\lambda )\right\rbrace ,\\= & \min \left\lbrace  \dim F^\perp ; \ F\in {F}^\mp _0(A;\lambda )\right\rbrace .\) 

